There is an automatic transmission equipped with a gear mechanism (e.g. planetary gear sets) having elements and a plurality of friction engaging means therefor for attaining a plurality of shift ranges by locking and releasing any one or more of the elements of the gear mechanism, which is disposed between an input shaft and an output shaft, to and from a case, by bringing the elements into and out of engagement with the input shaft or the output shaft, or into and out of engagement with each other. In order to achieve a down shift such as kickdown from a higher (e.g., third speed) to a lower (e.g., second speed) shift range the first fricton engaging means is brought into disengagement with the engagement of the second friction engaging means. This gear shift may involve a large shock if appropriate timing between the disengagement of the first friction engaging means and the engagement of the second friction engaging means is not controlled.
The timing between the engagement of the first engaging means and the disengagement of the second engaging means has heretofore been controlled by means of a one-way clutch in the automatic transmission, an accumulator in a hydraulic control system for the automatic transmission or a timing valve which controls the supply sequence of the working fluid to the hydraulic servos, all of which involve considerable complication of the mechanical structure of the automatic transmission and the hydraulic control system.